A diopter (D) is a unit of measurement for the optical power of a lens or curved mirror. It is equal to the reciprocal of the focal length measured in meters (1/meters).

How do I convert Focal Length to Diopters?

The formula is P = 1 / f, where P is power in diopters and f is focal length in meters. For example, a lens with a 0.5 meter focal length has a power of 2 Diopters.

How do I convert Prescription to Diopters?

To find the Spherical Equivalent (approximate total power), use the formula: Diopter = Sphere + (Cylinder / 2).

Does negative diopter mean near-sighted?

Yes. A negative diopter number (e.g., -2.50 D) indicates myopia (nearsightedness), meaning the eye is too strong or long. Positive numbers indicate hyperopia (farsightedness).

Diopter Calculator — Convert Diopters ↔ Focal Length

Instantly convert between diopters (D) and focal length (f).
Formula: P (D) = 1 / f (meters)

Diopters (D)

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Focal Length (f)

Calculate the new maximum focusing distance when adding a close-up lens (diopter) to your camera lens.

Close-up Lens Power (D)

Convert Sphere (Sph) and Cylinder (Cyl) to Spherical Equivalent (SE).

Sphere (Sph)

Cylinder (Cyl)

Enter your Sphere prescription. We’ll find the nearest standard swim goggle lens step.

Your Prescription (Sph)

Measure the furthest distance you can see text clearly (blur point) to estimate your myopia level.

Blur Distance

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Enter a value above

P = 1 / f

How is this calculated?

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Understanding Diopters and Focal Length

Whether you are a photographer focusing on macro shots, looking for prescription eyewear, or trying to select the right swim goggles, understanding diopters is essential. The Diopter Calculator helps you translate the abstract number of “optical power” into a physical distance you can measure.

What is a Diopter?

A diopter (symbol: D) is a unit of measurement for the optical power of a lens or curved mirror. Technically, it is the reciprocal of the focal length measured in meters.

Formula: P (Diopters) = 1 / f (meters)
Inverse: f (meters) = 1 / P (Diopters)

For example, a lens with a focal length of 1 meter has a power of 1 diopter. A lens with a focal length of 0.5 meters (50 cm) has a power of 2 diopters.

Diopters in Photography

In photography, “Close-up filters” are often rated in diopters. Attaching a +2 diopter filter to your camera lens sets your maximum focusing distance to 0.5 meters (50cm), regardless of the lens’s infinity focus. This is crucial for macro photography, allowing you to get closer to your subject.

Prescription Eyewear & The “Spherical Equivalent”

Eye prescriptions often contain three numbers: Sphere (Sph), Cylinder (Cyl), and Axis. If you are buying ready-made reading glasses or swim goggles, these products usually only come in “Sphere” steps. To find the closest match, optometrists use the Spherical Equivalent formula:

Spherical Equivalent = Sphere + (Cylinder / 2)

Myopia Estimation

Myopia (nearsightedness) can be roughly estimated by measuring the “far point”—the furthest distance you can read text clearly without glasses. If you can only read text clearly up to 20cm away, your focal length is 0.2m. Using the formula 1/0.2, your approximate myopia is -5.00 Diopters.

Frequently Asked Questions

What is the formula for calculating Diopters? +

The formula is P = 1 / f, where ‘P’ is Power in Diopters and ‘f’ is focal length in meters. Always ensure your focal length is converted to meters before dividing.

Can I convert contact lens prescription to glasses? +

Not exactly using this tool alone. Contact lenses sit directly on the eye, while glasses sit about 12-14mm away (vertex distance). For prescriptions under +/- 4.00D, they are very similar. For higher powers, a specific vertex conversion is needed.

What do positive and negative diopters mean? +

Negative numbers (e.g., -3.00 D) indicate a concave lens used to correct Nearsightedness (Myopia). Positive numbers (e.g., +2.00 D) indicate a convex lens used for Farsightedness (Hyperopia) or reading glasses (Presbyopia).

Why are swim goggles only available in steps of 0.5 or 1.0? +

Underwater vision does not require the same precision as air vision due to the refraction of water. Manufacturers produce “step” diopters (e.g., -2.0, -2.5) which are sufficient for clear vision while swimming.

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